pardon me for stating the obvious, but are you pressing the Fn key AND the brightness key?

in any case, try the following:

navigate to the Display Properties. select the Display tab, press the Advanced button.

on my machine there is a tab for the GeForce 8400 GS. Maybe your machine has a different video card, but there may be a similarly named tab for your video card. on this tab I have a button that says "Start the NVDIA Control Panel".

There is an option called Mobile, and an option on that page called Change PowerMizer Settings. There are two settings: